As consumer demand for high data rate applications, such as streaming video, expands, technology providers are forced to adopt new technologies to provide the necessary bandwidth. Multiple Input Multiple Output (“MIMO”) is an advanced radio system that employs multiple transmit antennas and multiple receive antennas to simultaneously transmit multiple parallel data streams. Relative to previous wireless technologies, MIMO enables substantial gains in both system capacity and transmission reliability without requiring an increase in frequency resources.
MIMO systems exploit differences in the paths between transmit and receive antennas to increase data throughput and diversity. As the number of transmit and receive antennas is increased, the capacity of a MIMO channel increases linearly, and the probability of all sub-channels between the transmitter and receiver fading simultaneously decreases exponentially. As might be expected, however, there is a price associated with realization of these benefits. Recovery of transmitted information in a MIMO system becomes increasingly complex with the addition of transmit antennas.
Many MIMO detection algorithms have been proposed. The maximum-likelihood detector, while conceptually simple and exhibiting optimal detection performance, is often impractical because its complexity increases exponentially with the number of input channels. Consequently, a variety of algorithms have been proposed to solve the detection problem with reduced complexity while sacrificing minimal performance. A summary of many previously proposed MIMO detectors is given in Deric W. Waters, Signal Detection Strategies and Algorithms for Multiple-Input Multiple-Output Channels (December 2005) (unpublished Ph.D. dissertation, Georgia Institute of Technology), http://etd.gatech.edu. In Bernard M. Hochwald & Stephan ten Brink, Achieving Near-Capacity on a Multiple-Antenna Channel, 51 IEEE TRANSACTIONS ON COMMUNICATIONS 389-99 (2003), the list-sphere detector was proposed as a way to compute the log-likelihood ratio (LLR) for the channel input. The list-sphere detector is one member of a large class of list detectors.
A MIMO system may employ orthogonal frequency division multiplexing (“OFDM”) to increase spectral efficiency, reduce frequency-selective fading, and combat inter-symbol interference (“ISI”) without computationally intensive equalization. OFDM partitions a high-speed data signal into a number of lower-speed signals that are transmitted in parallel over sub-divisions of the system bandwidth known as sub-carriers or tones. MIMO detection is applied to each of the tens or hundreds of sub-carriers of a MIMO-OFDM system. Therefore, efficient allocation of computational resources to MIMO-OFDM sub-carrier detectors is desirable. Note that the term OFDM as used herein also includes OFDMA systems where a given receiver only processes a subset of the tones in the OFDM symbol.